1. Field of the Invention
The present invention relates to an improved continuous duty comparator circuit. More particularly, the present invention relates to a low power comparator having a wide common mode range and high resolution, utilizing common digital CMOS processing technology.
2. Art Background
A comparator is a circuit that tells which of two inputs is larger. Comparators are used for a variety of applications. The simplest form of a comparator is the high gain differential amplifier constructed with transistors or operational amplifiers. A number of semiconductor chips exist on the market which perform the function of comparators. CMOS comparators are typically used in low voltage applications. However, due to fabrication sensitivities and manufacturing consumption constraints the resolution of typical comparators is limited to approximately 10 millivolts (mv). For example, in order to achieve true lmv resolution using a 3 volt power supply, a minimum of 3,000 steps is required along with a gain of 68 dB.
The main problem in developing a high gain comparator is that even though, ideally, it is possible to develop a very high gain comparator from simple digital stages such as inverters, the usable gain is only a small fraction of the ideal since offsets, which are realized at the output as measurement errors, of the individual stages are multiplied by the following stages. This implies that a precision analog circuit design implementation such as utilized by operational amplifiers is preferred. If analog circuit design techniques and layout methods are utilized, it is possible to design a circuit in which the individual stage offsets are small, and the offset contributions do not reduce the reflective offset at the input. Large gain is required since it is desirable to have the output switch to the supply rails so that the comparator can directly interface with a digital latch. In order for the comparator to convert 1 millivolt into 5 volts the gain must be greater than 5,000.
Some comparators can be characterized as a continuous duty comparator in which the two input signals are continuously compared to generate a differential output signal. However, the circuit is degraded by offsets incurred while performing the comparison. This is primarily due to manufacturing misalignments and gradients.
Another type of comparator is an offset corrected or clocked comparator in which the two inputs are periodically sampled and compared. To compensate for offsets, error compensation techniques are employed. For example, a capacitor is located at the output of the comparator to store the output. The charge stored in the capacitor is flipped in orientation and is subsequently discharged back into the comparator to cancel the offset. However, the charge injection from the capacitor and noise due to spikes is a constant problem. Also, noise injected by switching can present a significant problem. Furthermore, amplification of the output of the comparator amplifies the offset and increases the frequency of errors. Today's technology requires higher resolution, broad common mode range, no offsets and devices which consume little power. Thus, it is desirable to use a continuous duty comparator with little or no offsets and a wide common mode range.
In a system, such as an analog to digital converter (ADC), the comparator element typically causes the most significant contribution to the ADC error budget. The comparator typically determines the resolution as well as the switching speed and settling characteristic of an ADC. When evaluating error budgets, the digital to analog converter (DAC) contributes the second most significant contribution. From this system understanding it is clear that improvements in the comparator will significantly improve the ADC performance. The ADC in turn is used to provide a real world interface in order to perform comparisons to render decisions such as battery level voltage, ambient light power down conditions, etc. Thus, the overall performance and battery life of a system is to a large extent dependent on the efficiency and accuracy of the comparator.